Magnetic fine particles such as .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and fine particles thereof containing cobalt ions adsorbed thereon or doped thereinto have so far been used as magnetic particles for magnetic recording media. With the recent demand for high-performance video tapes and audio tapes which can record signals at a high density and reproduce short-wave recorded signals at high outputs, ferromagnetic metal fine particles of Fe or Fe-Co or ferromagnetic metal fine particles comprising Fe, Co, Ni, etc. have come into use for such tapes. However, these ferromagnetic metal fine particles tend to agglomerate since they have small crystallite sizes and, in addition, show very large magnetic moments. Because of such agglomeration tendency, it is extremely difficult to uniformly disperse these ferromagnetic metal fine particles into a binder.
For the purpose of overcoming the above problem by improving the affinity of a binder for magnetic particles, there has been proposed, for example, a binder having in its molecular structure a carboxyl group or a sulfonate group (JP-A-57-92422). (The term "JP-A"as used herein means an "unexamined published Japanese patent application".) However, since this binder is still insufficient in the ability to disperse ferromagnetic metal fine particles, not only the magnetic layer using this binder has poor surface properties and is insufficient in residual magnetic flux density and squareness ratio, but also the durability of the magnetic layer is so poor that a dust is apt to be generated therefrom.
JP-A-59-40320 proposes a magnetic recording medium which is excellent in the dispersibility of magnetic particles and the durability and surface gloss of the magnetic layer. This recording medium employs a combination of a vinyl chloride-type binder, which has a hydrophilic group introduced therein and is excellent in the ability to disperse magnetic particles, with a polyurethane-type binder. There is a description in this JP-A to the effect that the polyurethane-type binder in which a hydrophilic polar group has been introduced serves to improve the dispersibility of magnetic particles and the durability of the magnetic layer, while the vinyl chloride-type binder in which a hydrophilic group has been introduced serves to improve the dispersibility of magnetic particles and the surface gloss of the magnetic layer. Although these properties have been improved due to such a combination of binders, this prior art failed to improve the soiling of calender rolls and the clogging of heads. For improving the properties of vinyl chloride copolymers, JP-A-60-235814 proposes a binder obtained by copolymerizing vinyl chloride, a monomer having a strong acid group containing sulfur or phosphorus, and a monomer having an X-OH group (wherein X is an organic residue). According to the specification of this JP-A, the dispersibility of magnetic particles and the surface gloss of the magnetic layer are improved by the presence of a strong acid group, such as a sulfonic acid group, in the binder, and further the durability and running properties of the magnetic layer are improved because the crosslinking reaction of the copolymer with an isocyanate proceeds to a sufficient extent due to the presence of OH groups which are bonded to the main chain through the medium of the organic groups, which is different from the case of polyvinyl alcohol in which OH groups are directly bonded to the main chain. However, even where such a binder is used, the soiling of calender rolls and the clogging of heads cannot be sufficiently diminished.
Furthermore, in attempts to improve the dispersibility of ferromagnetic metal fine particles and to smooth magnetic layer coatings, there have been proposed a method in which a surfactant is incorporated, a method in which ferromagnetic particles are treated with a silicone oil or the like, and a method in which a silane coupling agent or the like reactive to a binder is used. However, sufficient effects cannot be obtained by any of these methods. Thus, there has been a desire for a binder which can more effectively disperse ferromagnetic metal fine particles to be used in magnetic recording medium for high-density recording.